chem 634 introduction to transition metal catalysis · basic organometallic mechanisms • to...
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Chem 634
Introduction to Transition Metal Catalysis
Reading: Heg Ch 1–2 CS-B 7.1, 8.2–8.3, 11.3
Grossman Ch 6
Announcements
• Problem Set 1 due Thurs, 9/24 at beginning of class
• Office Hour: Wed, 10:30-12, 220 BRL
Announcements
Organometallic Complexes
M0 III IV V VI VII VIII IX X XI XII d3 d4 d5 d6 d7 d8 d9 d10 d10s1 d10s2
1st row Sc Ti V Cr Mn Fe Co Ni Cu Zn 2nd row Y Zr Nb Mo Tc Ru Rh Pd Ag Cd 3rd row La Hf Ta W Re Os Ir Pt Au Hg early dividing line late
Rh ClPPh3PPh3
Ph3PPPh3Pd PPh3PPh3 PPh3
ClPt
ClCl Cl
2–
Ootol2P N
IrB
CF3
CF3 4Ph
Ph2P
PPh2
RuNH2
H2N Ph
Ph
Cl
Cl
How Many Valence Electrons?
E
2s
2p x 3
LCAO's of 4 H 1s orbitals
C 4 x HCH4
Bonding orbitals filled...stable configuration.
Consider CH4… Recall the Octet Rule
How Many Valence Electrons? Transition metals have 5 d orbitals (plus s & 3 p’s)!
E(n+1)s
(n+1)p x 3
M LxMLx
Bonding and nonbonding orbitals filled...stable configuration.
nd x 5
x M–L bonding orbitals
(9–x) nonbonding orbitals
x M–L antibonding orbitals
Determining Electron Count of TM Complex
Ionic Method: All ligands are dissociated with their electrons.
L-type ligands = neutral, 2 e- donor X-type ligands = anionic 2 e- donor
M
X
L
X
L
n
M
X
X
LLn+y
where y = number of x-type ligands
M
X
L
X
L
mnemonic:
M+
X
L
X–
L
Electron counting is a formalism.
Example of Ionic Method Electron Counting
Fe2+
H–
C OFe COH
COOCOC
HCO
CO
CO
H–
4 X CO 4 X 2e- 8e-
2 X H– 2 X 2e- 4e- Fe(2+) (d6) 6e- ________________________________________
18e-
Note: Both Ionic and Neutral Methods give the same electron count, but differ in how they treat the metal center.
Charged Complexes
4 X Cl- 4 X 2e- 8e-
Pd(2+) (d8) 8e- ________________________________________
16e-
ClPdCl
ClCl
2– 2K+
Oxidation State
• Formalism to account for “oxidation state” of metal center. • This is only a formalism, in reality electrons (and thus charge) are
shared over both the ligand and metal. • Nonetheless, oxidation state guides understanding of metal
reactivity. • Using ionic method of electron counting arrives directly at metal
oxidation state. • Example:
Fe COH
COOCOC
H
4 X CO 4 X 2e- 8e-
2 X H– 2 X 2e- 4e- Fe(2+) (d6) 6e- ________________________________________
18e-
Oxidation State = Fe(II)
Examples of X-type Ligands
hydride
halide
amido
alkoxide
M H
M Cl
H
Cl
M+ +
M+ +
M+ +
M+ +
-1
-1
-1
-1
2
2
2
2
M NR2
M OR
NR2
OR
alkyl
aryl (σ bound)
acetylide
silane
M+ +M CR3 CR3 -1 2
M M+ + -1 2
M R M+ + R -1 2
M SiR3 M+ + SiR3 -1 2
Ligand Complex Disconnection ligand e-ligand charge
Examples of X-type Ligands (More Complex)
acetate (η1)
acetate (η3)
allyl (η1)
allyl (η3)
M O
M
M
M
M+ +
M+ +
M+ +
M+ +
Ligand Complex Disconnection ligand e-ligand charge
-1
-1
-1
-1
2
4
2
4
Me
O
O
OMe
O Me
O
OMe
O
cyclopentadienyl (Cp) M M+ + -1 6
• 𝜼 (eta) denotes “hapticity” = how many atoms of ligand are bond to metal from single donor site.
Examples of 2e– Donor “L”-type Ligands
phopshine
amine
carbonyl
alkene
sigma bond (C–H)
M PPh3
M NEt3
M C
M
MH
CR3
O
PPh3
NEt3
C O
H
CR3
M +
M +
M +
M +
M +
Ligand Complex Disconnection ligand e-ligand charge
0
0
0
0
0
2
2
2
2
2
ether M OR2 OR2M + 0 2
sigma bond (H–H) MH
H
H
HM + 0 2
nitrile M N CR N CRM + 0 2
“L”-type Ligands Donating More than 2e–
• 𝜅 (kappa) denotes “denticity” = how many binding sites of a polydentate ligand are bound .
Ligand Complex Disconnection ligand e-ligand charge
M
Mdiene
arene (η6)
arene (η2)
M
M +
M +
M +
0
0
0
4
6
2
MP
PPh Ph
Ph PhP
PPh Ph
Ph Ph
M +2 X 2e– L-type ligandsbisphosphine (κ2)
0 4
MP
NR R
R RN
NR R
R R
M +2 X 2e– L-type ligandsdiamine (κ2)
0 4
X2 Ligands
oxo M O M2+ +
Ligand Complex Disconnection ligand e-ligand charge
-2 4O2-
imido (bent) M N M2+ + -2 4NR2-
triplet carbenes(alkylidines)
M CR2 M2+ + -2 4CR2
2-
R
NHC-Ligands
NNC
R
RM N
NCM
Sigma Bond Pi-Backbond
Ligand Complex Disconnection ligand e-ligand charge
N-heterocyclic carbenesN
NM
R
R
N
N
R
R
M +0 2
18 Electron “Rule”
Many, but not all, stable, isolatable TM complexes are 18e– complexes.
NOTE: 18e– “rule” is more of a suggestion. Many of the most interesting, i.e. reactive, complexes do not follow this “rule”.
Pd
PPh3
PPh3Ph3P
PPh3
Pd = group 1010e- + 8e- = 18e-
Ru
Cl
Cl
H2N
NH2
Ph2P
PPh2
Ru = group 8+2 = d6
6+12 = 18
Basic Organometallic Mechanisms
• To understand transition metal catalysis, we need to understand something about the mechanisms by which the catalysts operate.
• Many TM catalyzed reactions proceed via a series of elementary steps.
Dissociative Ligand Substitution
common with: • coordinately saturated • 18e- complexes
M
LL
LL
L
L
-LM
L
L
LL
L
L'M
LL
L'L
L
L
Associative Ligand Substitution
L'
L ML
LL'
L �L � M
L �
L �L �'L � M
L �
L �L �
common with: • 16 e- complexes • requires open coordination site
Y* ML �
L �L
L'Y* M
L �L'
Ltrans effect
equatorial ligand w/strongest pi-interactions
Oxidative Addition
[Mn] RX
[Mn+2]R
X
• formal oxidation state of metal center increases by two • favored by electron-rich metal centers
Example:
Ph3P Pd0Ph3P
Ph
I
Ph3P PdIIPh3P
Ph
I
Reductive Elimination
[Mn]R
R[Mn-2] + R R
• formal oxidation state of metal center decreases by two • promoted by electron poor metal centers • large ligands accelerate process • microscopic reverse of oxidative addition
N
NNiII
Ph
NO
N
NNi0 +
O
NPh
Example:
Transmetallation
[Mn]R
X
M' R'[Mn]
R
R'+ M' X
• Exchange of ligands between different metal centers • No change in oxidation state at metals • Exact mechanisms are complex, some are still unknown.
Ph3PPdII
Ph3P
Ph
IBu3Sn Ph Ph3P
PdIIPh3P
Ph
Ph+ Bu3SnI
Example:
Migratory Insertion
[Mn]R
X[Mn]
XR
• Ligand inserts into adjacent metal-ligand bond • No net change of oxidation state at the metal
Examples: PPh3
RhI
PPh3
OC CO
R
PPh3
RhI
PPh3
OCO
R
Ar
PdII
PPh3
Ph3P
+
PdII
PPh3
Ph3PAr
+
β-Elimination
[Mn]R [Mn]
R
• Microscopic reverse of migratory insertion • No net change of oxidation state at the metal • Most often observed as β-hydride elimination from alkyl ligand • Requires open coordination site on the metal center
PPh3
PdIIPh3P
+
Ar
Hnote opencoordination site
PPh3
PdII
H
Ph3P
+
Example:
β-Elimination
Ph
[Pd]H1H2
H3
H2
PhH3
Note: most of the time, β-hydride elimination is superfacial. This is because the mechanism is inner-sphere, involving formation of a metal-hydride bond.
PPh3
PdIIPh3P
+
Ar
Hnote opencoordination site
PPh3
PdII
H
Ph3P
+
α-Elimination
[Mn]R'H
R[Mn]
R
+ HR'
[Mn]R
R'
H
or: [Mn]
R' H
R ‡
Via:
not common in organic synthesis
Oxidative Coupling/Cyclization
[Mn] [Mn+2][Mn]
‡
σ−Bond Metathesis
[M] R R' R'' [M] R'+
R R''
[M] R
R' R''
‡ [M] R
R' R''
Via:
Common mechanism of C-H bond activation.
π−Bond Metathesis
[M] RX Y
[M] X R Y+
via:
[M] R
X Y
[M] R
X Y
[2+2] [retro 2+2] [M] R
X Y
Common in alkene metathesis.